Avalanche photodiodes (APDs) detect incident optical photons and amplify the resultant charge generated, typically by an order of magnitude or more. See, for example, B. Saleh and M. Teich, Fundamentals of Photonics (1991), pp 666-673, which is incorporated herein by reference. APDs have use in nuclear imaging applications, for example, gamma cameras and other devices in which the nature of the radiation imaged is such that not many optical photons are generated by the interaction of the incident radiation and the scintillator material. Crystalline silicon (Si) is commonly used in the fabrication of APDs due to the advantageous photoelectric characteristics of silicon and the relative ease of fabrication.
APDs typically include an island or body of layers of appropriately doped crystalline silicon disposed between two electrodes electrically coupled to opposite surfaces of the APD body. Relatively high voltages (e.g., about 1000 V to 1500 V) are applied across the APD to generate the electric field necessary to cause the avalanche effect. As incident radiation is absorbed in the silicon, holes and electrons are produced and accelerate toward the upper or lower surface of the photodiode dependent on the electric field established by the electrodes, and as they accelerate they displace more holes and electrons, causing the amplification of the signal. The amount of charge collected at the electrodes is a function of the energy flux of the incident light and the amplification of the APD. Periodic measuring of charge collected on a photodiode, and resetting the diode to a known charge condition, is used to process electrical signals generated by the photodiode in response to incident radiation.
Noise in an array of APDs is critical because of the high electric fields and the consequent amplification of any noise-inducing factor. Noise arises from bulk leakage current, device capacitance, and charge injection into the high (electric) field region of the device from the surface of the APD. Because surface conditions are difficult to control in semiconductor processing, the APD must be structured so as to reduce the peak surface electric field well below the peak bulk electric field for stable device operation. This structuring is accomplished by beveling the periphery of a discrete device, or, alternatively, the mechanical grooves that provide isolation of individual APDs in a monolithic array of the devices. The beveling process results in a structure in which the doped material on one side of the p-n junction has been preferentially removed, leaving a beveled edge that slopes away from the vicinity of the p-n junction. Subsequent to beveling, the silicon surface must be treated with a suitable passivant to prevent charge injection from the surface and to limit the surface leakage current, thereby permitting low noise operation.
It is desirable that the passivating layer have the following characteristics: First, it should act as an electrically insulating barrier between the upper electrode, the APD body, and other underlying electrically conductive components that are used in reading and processing electrical signals generated by the photodiode. Second, the passivating layer should be capable of being selectively etched with respect to the silicon body of the APD to enable formation of a non-leaky contact connection between the upper electrode and the upper surface of the photodiode body. Third, the passivating layer should cover the photodiode body without cracking or inducing stresses that adversely effect photodiode performance or the dielectric integrity of the passivating layer. Fourth, the interface between the passivating layer and the beveled edge of the photodiode body should have minimal conductivity so that photodiode leakage in reverse bias is not degraded by the presence of the dielectric. Fifth, the passivating layer should be thick enough (on the order of about 5 microns) so that the electric field inside the dielectric does not become very large (e.g., less than about 2.times.10.sup.6 V/cm). Additionally, the passivating layer should desirably protect the APD beveled surfaces from degradation due to humidity, moisture, or chemical attack from materials in the environment or present on the wafer. This latter concern is present both during fabrication and also over time as the device is exposed to a variety of environments.
Passivating layers are commonly made up of one layer of material. Certain polyimides, particularly preimidized polyimides, have been found to provide a satisfactory passivating layer with regard to several of the desirable characteristics listed above. Drawbacks to polyimide passivating layers include the poor moisture barrier provided by polyimides. Indeed, most polyimides providing otherwise satisfactory passivating layer characteristics are hygroscopic, that is they tend to absorb moisture from the environment. This characteristic is particularly undesirable in light of the increased leakage resulting from degradation of the APD beveled surface in contact with polyimide. One manner of providing moisture protection has been to hermetically seal the entire chip containing the array of photosensors, a process that leads to bulkier arrays and increased steps in fabrication of the arrays.
Single layer inorganic dielectrics have been used in some situations, but typically such layers cannot be deposited to the required thickness to provide the desired passivating characteristics without experiencing debilitating stresses that affect the structural integrity of the dielectric layer and degrade device performance and longevity.
A two-layer dielectric structure is disclosed by R. Kwasnick and J. Kingsley in the application entitled "Photosensitive Element with Two Layer Passivation Coating", Ser. No. 07/891,117, filed Jun. 1, 1992 and allowed Feb. 3, 1993, now U.S. Pat. No. 5,233,181, which is assigned to the assignee of the present invention and incorporated herein by reference. The two-layer passivation structure comprises an organic dielectric layer and an inorganic moisture barrier layer. In devices comprising amorphous silicon, however, the inorganic dielectric layer is advantageously disposed adjacent to the amorphous silicon and the polyimide dielectric disposed over the inorganic layer in order to prevent moisture leaching from the polyimide into the amorphous silicon. Further, in devices in which amorphous silicon comprises the photosensitive semiconductive material, constraints of temperature in the formation process relative to the optimal cure temperature for the polyimide make it undersirable to place the polyimide immediately adjacent to the photosensitive material. Specifically, the maximum temperature to which the amorphous silicon structure can be exposed in the fabrication process without damaging the semiconductor material is about 250.degree. C., thus limiting the cure temperature of the polyimide and preventing all moisture from being driven out of the polyimide. Thus the moisture inherent in the polyimide itself contaminates the amorphous silicon structure.
It is thus an object of this invention to provide an APD having a passivating layer that provides a high integrity moisture barrier.
It is a further object of this invention to provide a passivating layer for an APD that provides a high integrity dielectric layer that is structurally sound.
Another object of this invention is to provide a low noise APD.